# How to calculate effective resolution of ciruit

Discussion in 'General Electronics Chat' started by embedded.world, Jun 10, 2014.

1. ### embedded.world Thread Starter Member

Feb 27, 2014
36
0
I am using 24 bit adc ADS1247 to measure voltage drop across thermsistor by passing current through it.
http://www.digikey.com/product-detail/en/PR103J2/615-1069-ND/2651611

Since thermistor value gets very large so I connected a 20K fixed resistor in parallel with it.

1. Now my current source is 50uA with initial error is +-6% with temperature drift +-100ppm/C.

2. Voltage reference is 2.048V with +-10mV & drift is +-15ppm/C.

3. Resistor = 20K +-0.01% with drift +-5ppm/C.

4. PGA = 2, operating range = -40C to +60C.

5. Minimum voltage step need to measure = 75uV.

Questions:
1. How to calculate effective resolution I will get finally.

2. Do I need any other factors to consider besides I mentioned above.

3. Do final resolution is equal to ENOB?

4. Will I be able to achieve minimum voltage step = 75uV with this adc

Crosspost:
http://www.eevblog.com/forum/beginners/how-to-calculate-effective-resolution-of-ciruit/

Last edited: Jun 10, 2014
2. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,298
225
Why don't you use some of the linearizion IC's, like TI's PGA309?

3. ### dougc314 Member

Dec 20, 2013
38
11
The thermistor has a non-linear resistance change with temperature. In order to calculate resolution you will need to generate on expression for resistance as a function of temperature. There are good articles on Wikipedia for this. Also most Thermistor vendors publish a table of resistance vs temperature for their thermistors and you could simply use the table to determine the dR/dT at various temperatures. I think the most important AD parameter you are looking for is differential non-linearity, check that out on Wikipedia as well